Willy, sorry to hear that this was a surprise and that it negatively affected your site performance. This is part of the opt-in data collection.

If you go to your wp-admin's YARPP settings page, you'll see the "Send settings and usage data back to YARPP" option. Turn this off and the pixel disappears. The "learn more" button there also tells you about the pixel image.

As for the performance of the image serving itself, it's definitely something that I've been improving over time.